Abdominal Imaging

, Volume 38, Issue 2, pp 367–375

Aortoenteric fistulas: spectrum of CT findings

Authors

    • Department of RadiologyJohns Hopkins University
  • Aya Kamaya
    • Stanford University
  • Michael Federle
    • Stanford University
  • Elliot K. Fishman
    • Department of RadiologyJohns Hopkins University
Article

DOI: 10.1007/s00261-012-9873-7

Cite this article as:
Raman, S.P., Kamaya, A., Federle, M. et al. Abdom Imaging (2013) 38: 367. doi:10.1007/s00261-012-9873-7

Abstract

Objective

This article reviews the causes of aortoenteric fistulas, diagnostic options, and important CT findings.

Conclusion

Aortoenteric fistula, a rare but potentially fatal entity, presents a significant challenge to radiologists in diagnosis, largely because of its subtle and nonspecific imaging findings. These fistulas can be divided into primary and secondary forms, depending on the presence or absence of prior aortic reconstructive surgery, but the secondary form is more common. Typical CT findings, which can overlap with those seen in perigraft infection, aortitis, infected/mycotic aneurysms, perianeurysmal fibrosis, and the immediate post-operative period after placement of a graft, include: Effacement of the fat planes around the aorta, perigraft fluid/soft tissue thickening, ectopic gas, tethering of adjacent thickened bowel loops towards the aortic graft, and in rare cases, extravasation of contrast from the aorta into the involved segment of bowel.

Keywords

Computed tomographyAortoenteric fistulaPerigraft infectionAortitis

A rare (seven cases of primary aortoenteric fistulas/100 million), life-threatening condition, aortoenteric fistulas present a significant diagnostic challenge [1, 2]. Early diagnosis is critical, as mortality in the absence of intervention is virtually 100% [3]. While endoscopy and other imaging modalities do have a role, the diagnosis of a fistula is critically dependent upon CT, which has a number of suggestive, albeit subtle, findings [4].

Due to the rarity of the disorder, there are fewer than 300 reported cases in the literature, and very few systematic descriptions of the key CT findings [3]. In this pictorial essay, we present 10 patients with aortoenteric fistulas, each of whom demonstrate varying suggestive findings on CT. As these cases demonstrate, the CT findings can be quite difficult to discern without a high index of suspicion.

Clinical presentation and etiology

Aortoenteric fistulas can be divided into primary and secondary forms: Primary fistulas, considered rare, occur in a native aorta without a history of prior intervention. While typically caused by an atherosclerotic penetrating ulcer, other causes can include diverticulitis, foreign bodies, aortitis, appendicitis, and gastrointestinal malignancies [58].

Secondary fistulas are seen in the setting of prior surgery or intervention, and are more common, with a reported incidence as high as 0.6% in patients with previous aortic surgery or graft placement [3, 9]. Patients at particular risk include those who undergo emergent surgery for a ruptured aneurysm, have post-operative complications such as reoperation or bowel injury, and those with endoleaks or stent migration [10]. The cause of secondary fistulas is thought to be the result of chronic perigraft infection or prolonged pressure upon the bowel by a graft [11].

While fistulas can occur with any part of the gastrointestinal tract, the classic location is the transverse portion of the duodenum, which is involved in 60% of cases. Less common locations include the remainder of the duodenum, jejunum and ileum, stomach, sigmoid colon, and ascending/descending colon, each of which is involved in less than 5% of cases [5].

CT findings

The most important CT finding in an aortoenteric fistula is ectopic gas either within or directly adjacent to the aortic lumen (Figs. 1, 2, 3, 4, 5, 6). In the most obvious cases, a direct tract of gas can be traced directly from the involved bowel loop towards the aorta. However, while highly suggestive, the presence of gas is not completely specific. Ectopic gas can be a normal finding in the first month after aortic reconstruction, and should not be confused with a fistula. Moreover, gas can also been seen in severe perigraft infections, even without the presence of a fistula. The most specific sign, the direct extravasation of contrast from the aorta into a bowel loop, is quite rare to identify on CT (Fig. 7). Similarly, the leakage of enteric contrast directly into the periaortic space is a highly specific sign, but extremely rare [1].
https://static-content.springer.com/image/art%3A10.1007%2Fs00261-012-9873-7/MediaObjects/261_2012_9873_Fig1_HTML.jpg
Fig. 1

A 65-year-old female with a history of multiple open abdominal aortic aneurysm (AAA) repairs and revisions (most recently 5 years prior), who presented with fever and hypotension (but no gastrointestinal bleeding or abdominal pain). A Axial and B coronal CT images demonstrate soft tissue thickening and fluid surrounding the aorta, a tethered adjacent duodenum, and ectopic gas (arrows) in the periaortic space. The patient subsequently died of sepsis.

https://static-content.springer.com/image/art%3A10.1007%2Fs00261-012-9873-7/MediaObjects/261_2012_9873_Fig2_HTML.jpg
Fig. 2

A 72-year-old male with a history of several aortic reconstructions, as well as prior ureteral reconstruction after surgical repair of an aortic pseudoaneurysm a few months prior. A Axial, B sagittal, and C coronal. CT images acquired during routine follow-up demonstrate a large amount of air (closed arrows) surrounding the aortic graft, possibly tracking from the adjacent duodenum, periaortic inflammatory change, as well as gas within the right intrarenal collecting system and bladder (not shown). An aortoduodenal fistula was confirmed at surgery, as well as fistulization with the right ureter. The patient survived without complication.

https://static-content.springer.com/image/art%3A10.1007%2Fs00261-012-9873-7/MediaObjects/261_2012_9873_Fig3_HTML.jpg
Fig. 3

A 63-year-old male with a history of a complicated surgical juxtarenal aortic aneurysm repair and endograft repair of iliac artery aneurysms presented with severe back pain. Axial (B, C) and sagittal (A) CT images demonstrate a small amount of perigraft ectopic gas (arrows) directly adjacent to the duodenum, which appeared tethered to the aorta. An aortoduodenal fistula was confirmed at surgery, and the patient died during operative repair.

https://static-content.springer.com/image/art%3A10.1007%2Fs00261-012-9873-7/MediaObjects/261_2012_9873_Fig4a_HTML.jpg
https://static-content.springer.com/image/art%3A10.1007%2Fs00261-012-9873-7/MediaObjects/261_2012_9873_Fig4b_HTML.jpg
Fig. 4

A 76-year-old man with a history of prior aorto-bi-iliac graft placement, who presented with recurrent fevers. Axial (A), sagittal (B), and coronal (C) CT images demonstrate an abnormal, cork-screw, distorted appearance of the aorta, with a tiny focus of adjacent ectopic gas (arrows) in between the graft and the closely adherent duodenum. An aortoduodenal fistula was confirmed at surgery, and the patient underwent uncomplicated operative repair.

https://static-content.springer.com/image/art%3A10.1007%2Fs00261-012-9873-7/MediaObjects/261_2012_9873_Fig5_HTML.jpg
Fig. 5

A 46-year-old female with an aortoenteric fistula thought to be related to retroperitoneal lymph node dissection and small bowel resection for recurrent metastatic colon cancer. Axial CT (A) images through the mid abdomen show ectopic gas (arrows) and fluid adjacent to the aorta. Follow-up angiogram (B) shows frank extravasation of arterial contrast into the small bowel (arrows). The patient died 6 weeks later.

https://static-content.springer.com/image/art%3A10.1007%2Fs00261-012-9873-7/MediaObjects/261_2012_9873_Fig6_HTML.jpg
Fig. 6

A 71-year-old female, who had undergone placement of an aorto-bi-iliac graft 4 years prior, presented with sepsis. Axial (A, B) noncontrast CT images demonstrate the thickened transverse portion of the duodenum directly tethered to the patient’s aortic graft, with a small amount of gas (arrows) within the graft itself. She underwent surgical repair of an aortoduodenal fistula, but had a complicated post-operative course with left lower extremity ischemia, subsequent amputation, and colonic ischemia. The patient died during surgery for her ischemic bowel.

https://static-content.springer.com/image/art%3A10.1007%2Fs00261-012-9873-7/MediaObjects/261_2012_9873_Fig7_HTML.jpg
Fig. 7

A 85-year-old male with a history of a prior open surgical abdominal aortic aneurysm repair, who presented with massive lower GI hemorrhage. Axial contrast-enhanced CT images demonstrate A a large right iliac artery aneurysm (open arrow), tethering of the sigmoid colon to the iliac aneurysm, and B direct contrast extravasation (arrows in B) from the aneurysm into the sigmoid colon. Notably, although oral contrast was administered, it had not yet reached the sigmoid colon. The patient died of massive hemorrhage shortly after the CT.

Ultimately, making the diagnosis of a fistula is dependent on a number of other nonspecific, but suggestive findings, including effacement of the periaortic fat plane, focal thickening and tethering of a bowel loop immediately adjacent to the aorta, periaortic free fluid and soft tissue thickening, and disruption of a graft or significant graft migration (Figs. 8, 9). In a series by Hagspiel, bowel wall thickening, effacement of the periaortic fat plane, and periaortic soft tissue/fluid were seen in virtually every patient, while ectopic gas was seen in only 56%. Notably, the extravasation of contrast from the aorta into the bowel lumen was present in only 11% of cases [11].
https://static-content.springer.com/image/art%3A10.1007%2Fs00261-012-9873-7/MediaObjects/261_2012_9873_Fig8_HTML.jpg
Fig. 8

A 92-year-old male with an aortoenteric fistula. Patient presented with hematemesis, upper GI bleeding and a large aortic aneurysm. He was thought to have aortic graft material directly eroding into the duodenum on endoscopy. Axial contrast-enhanced CT shows loss of the fat plane between the aorta and the transverse portion of duodenum (arrow), which appears tethered and minimally thickened. Inflammatory changes are also noted surrounding the aorta near its point of contact with the bowel, with minimal periaortic soft tissue prominence, but no ectopic gas. The patient underwent surgical exploratory laparotomy and was confirmed to have an aortoenteric fistula, which was repaired.

https://static-content.springer.com/image/art%3A10.1007%2Fs00261-012-9873-7/MediaObjects/261_2012_9873_Fig9_HTML.jpg
Fig. 9

A 73-year-old male with an aortoiliac graft (placed 19 years prior) who presented with upper GI hemorrhage. Axial contrast-enhanced images (acquired prior to the MDCT era) demonstrate the third portion of the duodenum directly tethered to the aorta, with abnormal periaortic soft tissue thickening and fluid (arrows). An aortoenteric fistula was confirmed during open surgical repair. However, the patient died of sepsis soon after surgery, with an extremely complicated post-operative course.

The nonspecific nature of these findings leads to considerable overlap with a number of other disorders. Of these, perigraft infection is the most important, as it can look identical to a fistula. The clinical history (especially gastrointestinal bleeding or hematemesis) should raise diagnostic suspicion for a fistula, and while ectopic gas can be seen in both disorders, it is more common with the presence of a fistula. Secondly, the CT features of a fistula can be normal findings in the immediate post-operative period, and ectopic gas (normal up to 1 month) and perigraft fluid (normal up to 3 months) should not be misinterpreted as a fistula. Other potentially similar appearing entities include aortitis, mycotic aneurysms, and perianeurysmal fibrosis, all of which can demonstrate periaortic inflammation, fluid, or soft tissue [1].

Identifying a primary fistula can often be more difficult, given that the index of suspicion is typically lower. A penetrating ulcer or focal intramural hematoma in close proximity to a tethered, abnormal appearing loop of bowel should raise suspicion, as penetrating ulcers are the most common cause of a primary fistula. Furthermore, focal hematoma can also be seen in the wall of the affected bowel segment, or alternatively, between the bowel and aorta (Fig. 10) [11].
https://static-content.springer.com/image/art%3A10.1007%2Fs00261-012-9873-7/MediaObjects/261_2012_9873_Fig10_HTML.jpg
Fig. 10

A 72-year-old female with no prior history of surgery, who presented with upper GI hemorrhage. A Initial noncontrast axial CT demonstrates a large abdominal aortic aneurysm, with focal crescentic high density hemorrhage (arrow) within the periphery of the thrombosed portion of the aneurysm sac, immediately adjacent to a tethered-appearing duodenum (open arrow). B Subsequent contrast-enhanced axial CT image demonstrates ectopic gas (arrow) within this crescentic high density area. An aortoenteric fistula was identified during surgical exploration and repair.

Ancillary diagnostic findings

Endoscopy can exclude other causes of significant upper GI bleeding, although it is not uncommon for an incidental ulcer to be mistaken as the primary cause of bleeding [2, 12]. Moreover, the fistula itself is rarely seen by endoscopy, and the diagnosis is often only hinted at by the presence of blood in the duodenum. Only 25–62.5% of aortoenteric fistulas could be diagnosed using endoscopy in one series [2].

While many assume that conventional angiography is the gold-standard in diagnosis, angiographic diagnosis is actually quite difficult, as most of these cases have slow or intermittent flow into the fistula (Fig. 5) [4]. Nuclear medicine Technetium-labeled red blood cell (RBC) scans are also limited, with poor spatial resolution and a lack of specificity [13].

Management

Classically, the treatment for aortoenteric fistulas involved resection of the infected graft, bowel resection, and creation of an extra-anatomic bypass graft [6, 10]. Unfortunately, mortality rates are extremely high (up to 90%), and there is a high rate of limb amputations (up to 50%) and aortic stump disruptions (up to 38%) [10, 14]. Over the last decade, the trend has been towards the use of endovascular techniques for both primary and secondary fistulas. The use of endovascular techniques eliminates the most common fatal complication of open surgery—dehiscence of the aortic stump, and also has a lower risk of perioperative complications [10, 14].

Conclusion

As the cases we have presented demonstrate, the CT features of aortoenteric fistulas can be quite subtle. While the CT findings are often not completely specific, the burden of diagnosis still falls upon CT, as other modalities such as endoscopy, nuclear medicine studies, and angiography have significant limitations. Ultimately, a high index of suspicion is required given the potentially fatal consequences of a missed diagnosis.

Copyright information

© Springer Science+Business Media, LLC 2012